High-power laser systems (for example, laser systems employing a master-oscillator power-amplifier (MPOA) configuration) are subject to damage if a back-reflected signal from a distal end of a delivery fiber re-enters the power-amplifier stage. Typically, circular fibers are used as delivery fibers for pulsed and continuous-wave (CW) laser signals. However, it has been shown (M. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses”, OPTICS LETTERS/Vol. 23, No. 1/Jan. 1, 1998, which is incorporated herein by reference) that the fundamental mode in a fiber can propagate in multimode fibers over long distances. Therefore, when a reflection occurs after the laser light exits the delivery fiber, laser light can propagate backwards in the delivery fiber and maintain or nearly maintain its beam quality. Consequently, reflected signals can be coupled into the amplifier and lead to damage of the amplifier or laser. To prevent this, conventional systems typically employ an optical isolator that allows only one-directional signal propagation in a direction away from the power amplifier. Such isolators are expensive and complex.
U.S. Pat. No. 7,576,909 issued to Harter, et al. on Aug. 18, 2009 titled “Multimode amplifier for amplifying single mode light,” and is incorporated herein by reference. Harter et al. describe techniques for the control of the spatial as well as spectral beam quality of multi-mode fiber amplification of high-peak-power pulses, as well as using such a configuration to replace diode-pumped, Neodynium based sources. Harter et al. assert that perfect spatial beam-quality can be ensured by exciting the fundamental mode in the multi-mode fibers with appropriate mode-matching optics and techniques. The loss of spatial beam-quality in the multi-mode fibers along the fiber length can be minimized by using multi-mode fibers with large cladding diameters. Near diffraction-limited coherent multi-mode amplifiers can be conveniently cladding pumped, allowing for the generation of high average power. Moreover, the polarization state in the multi-mode fiber amplifiers can be preserved by implementing multi-mode fibers with stress producing regions or elliptical fiber cores.
U.S. Pat. No. 7,590,323 to Broeng et al. issued Sep. 15, 2009 titled “Optical fibre with high numerical aperture, method of its production, and use thereof” and is incorporated herein by reference. Broeng et al. describe an optical fiber, having at least one core surrounded by a first outer cladding region, the first outer cladding region being surrounded by a second outer cladding region, the first outer cladding region in the cross-section comprising a number of first outer cladding features having a lower refractive index than any material surrounding the first outer cladding features, wherein for a plurality of said first outer cladding features, the minimum distance between two nearest neighboring first outer cladding features is smaller than 1.0 μm or smaller than an optical wavelength of light guided through the fiber when in use; a method of its production, and use thereof. They also describe fibers built from performs having non-circular tubes or rods that form non-circular cores for the collection of light from laser diodes having non-symmetric non-circular beams.
A paper in the 30 Oct. 2006 Vol. 14, No. 22 of OPTICS EXPRESS pages 10345-10350 by J. R. Hayes et al. titled “Square core jacketed air-clad fiber” is incorporated herein by reference. Hayes et al. describe fabrication of a highly multi-mode square core jacketed air-clad fiber with a top-hat near-field intensity profile, and using this fiber to deliver Q-switched pulses to ablate square marks on indium tin oxide films.
There is a need for improved laser systems, particularly optical-fiber lasers and/or optical-fiber amplifiers having delivery fibers, wherein systems have improved optical isolation to prevent optical feedback due to reflections in the delivery fiber or at its ends from traveling back into, and damaging, the optical-fiber lasers and/or optical-fiber amplifiers. There is also a need to eliminate a conventional isolator to reduce the cost, size and complexity of the system.